1. Field
This invention relates to an apparatus and method for generating high-energy neutrons. The invention particularly relates to a neutron generating target apparatus and method for producing a high flux of high-energy neutrons.
2. Prior Art
High-energy neutrons may be generated by the T(d,n)He.sup.4 reaction which is well known to the skilled artisan as the D-T reaction. This reaction is typically produced by causing deuterium ions from an accelerator to impinge on a titanium target which is impregnated with tritium. As the deuterium ions bombard the tritiated target, neutrons are emitted at the rate of approximately one for every 10.sup.5 ions impinging on the target.
As ions contact the target in the D-T reaction, almost all of the kinetic energy of said ions is transformed into thermal energy therein. If the temperature of the neutron emitting tritiated target is allowed to exceed a few hundred degrees Centigrade, the tritium diffuses out of the target, thus impairing the neutron-producing efficiency of the target and decreasing its useful life. Thus, cooling of the target material becomes critical to prolonging the life of the target. The cooling problem in the D-T reaction is aggravated by the fact that the target material is typically a poor conductor of heat and that the side of the target material in which the energy is deposited is in a vacuum.
Another factor which limits the flux and reduces the lifetime of targets which use the D-T reaction is what might be called the "dilution effect". This is the effect of cumulative implantation of projectile ions into the target during bombardment, whereby these implanted ions dilute, and eventually displace the target atoms, thereby diminishing the yield of the target.
In the prior art, stationary and geometrically flat targets have been used as neutron sources. These neutron sources have generally short lives as a result of rapid deterioration due to overheating and dilution of the target. Recently, methods for utilizing dynamic targets have been disclosed with reported success. For example, see Booth, R. "Rotating Neutron Target System" UCRL-70183, University of California Radiation Laboratory, February 1967; Booth, R. and Barschall, H., "Tritium Target for Intense Neutron Source," UCRL-73525, University of California Lawrence Radiation Laboratory, November 1971, which disclose rotating, water-cooled targets which yield approximately 2 times 10.sup.12 neutrons per second at the source for a lifetime in excess of 100 hours, using a 400 KEV beam of deuterons (atomic deuterium ions) of 8 milliamperes or 3200 watts. See also D. D. Cossuta, "Target Technology for Medium and High-Power Applications," Proceedings of the Second Oak Ridge Conference on the Use of Small Accelerators for Teaching and Research, Mar. 23-25, 1970, CONF-700322.
The use of high-energy neutrons in the irradiation of cancers is finding increasing acceptance as an effective therapeutic technique. The theoretical and clinical work performed to date indicate strongly the need for a collimated beam of neutrons of about 14 MEV at an intensity or flux level at the neutron source, assuming isotropic emission, of at least 4 times 10.sup.12 neutrons per second, for a target lifetime in excess of 100 hours, in order to provide effective therapeutic irradiation of a patient situated at a distance of about one meter from the neutron source. Thus, even the improved targets of Booth and Barschall do not provide a sufficiently high intensity neutron source for cancer therapy and other high neutron intensity applications.
In applicant's copending application "Neutron Generator Target Assembly", Ser. No. 286,402, filed Sept. 5, 1972, now U.S. Pat. No. 3,860,827, which is incorporated herein by reference, it is shown that in applications of neutron generators to the production of a collimated beam of neutrons, as required for cancer therapy, it is useful to have a neutron source area which is not circular or square, as in the target apparatus in the references of Booth, Barschall and Cossuta, but is elongated in the direction in which neutrons produced in the target are designed to be emitted from the target apparatus. In applications using a neutron collimator, the desired elongation is in the direction substantially along the axis of the neutron collimator. Such elongation results in increasing the area on which the ion beam impinges, thereby diminishing the heat load per unit area of the target and reducing the dilution effect. Further, an elongated neutron source does not impair the quality of the collimated beam. The prior art has shown that effective collimation can be achieved with a source of neutrons whose area, projected on a plane perpendicular to the axis of collimation, does not exceed 2 cm by 2 cm. This requirement can be achieved with a source viewed end-wise of generally rectangular area whose width is considerably greater than 2 cm. Applicant in his copending application Ser. No. 286,402 sought to utilize this feature. However, the neutron source was elongated in directions which were parallel or anti-parallel to the direction of the ion beam as well as the collimator axis. This arrangement required that neutrons pass through the target support and coolant on their way to the patient, causing scattering and absorption of said neutrons, thereby diminishing the efficiency of neutron transmission. Also, in anti-parallel emission, means are required to avoid scattering of neutrons by the accelerator itself. These means complicate the problem of effectively collimating the neutron beam in the direction of the patient.
As used in the description and claims herein the long dimension of the generally rectangular neutron source shall be referred to as the width of the source and the other dimension as the height.
The prior art has shown that the useful life of a target can be prolonged by having impinging ions of a single mass species. See, for example, Booth and Barschall cited above. They have noted that the ion sources commonly used in accelerators produce both atomic (D.sup.+) and molecular (D.sub.2.sup.+) ions, often in comparable quantities, and that the shorter range D.sub.2.sup. + ions are implanted in the target at depths which dilute the target atoms for the D.sup.+ ions. In the prior art, the molecular ions have been sorted out and discarded, thus leaving the atomic ions alone as the one species of target-bombarding particles. The prior art method for discarding unwanted ions includes the use of mass analysis in the high voltage terminal of the accelerator so that the unwanted ions would not present a problem after emission from the accelerator. The above method is known in the art as "terminal analysis" and is considered to be a problem since it is expensive and degrades the quality of the ion beam which is selected for acceleration. Also, the combination of molecular and atomic ions produced by conventional ion sources is used inefficiently in known methods since all ions are not used to bombard the target in order to cause neutron generation.
Other prior art references which are of a more general interest are U.S. Pat. No. 2,251,190 to Kallmann, U.S. Pat. No. 2,929,933 to Ela et al., U.S. Pat. No. 2,712,081 to Feason et al., U.S. Pat. No. 2,943,239 to Goodman, U.S. Pat. No. 3,311,769 to Schmidllein, and U.S. Pat. No. 2,576,600 to Hanson.